Question: How do I know if the gap is not optimal? Is it worth having adjusted?

GR : The gap is not optimal if the horn does not slot or it feels too resistant. I have a simple formula to use for setting the gap. This has never failed me.

1. Measure the exit wall of the mouthpiece. It is usually about .020" to .030".

2. Then measure the step in the receiver and leadpipe (effective leadpipe wall).

3. Multiply the exit wall of the mouthpiece by 5 and the leadpipe wall by 1.5. Example, .022 mouthpiece wall X 5 = .110". Now the leadpipe, .020" leadpipe wall x 1 .5 = .030".

4. Add the .110" and the .030" and you get an optimal gap of .140".

If you are +/- .025" things should still work fine. If you go much more a bit of resistance will be added but the horn will speak and slot. If the gap is too little slotting can be a problem and the horn will feel less resistant. It is definitely worth having the gap adjusted.